While i worked with the MPTM format, found out about some wierd overwriting effect if an instrument is paned and other instruments (that are not paned) become paned the same way in playback as if i put an Xxx command in row 0 (which i didn't).
I put here an example file of this happening (its happening in pattern 4 at channel 3).
You have to think of panning as a channel property. If a sample or instrument doesn't provide a default panning value, the previous panning value is always used.